This invention relates to electrical switchgear. More specifically, it relates to electrical switchgear with arc fault light detectors.
In the past, several products have been designed to detect the light caused by an arc in electrical switchgear and terminate that arc as quickly as possible. These products operate by detecting a magnitude of light above a fixed threshold to determine the presence of an arc. When the presence of an arc is detected, the circuit indicates such and a mechanism is actuated to remove the source voltage.
Unfortunately, an arc does not produce light with a high content of radiation in the infrared frequency range. This is the range most easily detected by conventional light detectors, such as photodiodes and phototransistors. Therefore, the signal detector has to be set so as to be very sensitive in order to detect an arc before it becomes too destructive.
However, flashlights, sunlight, and building light sources have a much greater infrared content. Even though the absolute level of light from these sources is typically much lower, when conventional light detectors are subjected to these sources it can cause an erroneous detection of a arc. The source voltage is then erroneously tripped and must be reset. This erroneous detection is exacerbated by the need to set the detector to be very sensitive.
Therefore, there is a need for an improved switchgear arc fault detection circuit. The present invention is provided to solve this and other problems.
The present invention provides an improved electrical circuit for detecting an arc fault in electrical switchgear. To this end, there is provided a circuit which comprises a light detector which generates a light detector signal in response to the amount of light detected. A differentiator circuit receives the light detector signal and differentiates the light detector signal over time to generate a differentiated light detector signal. A comparator circuit then compares the differentiated light detector signal to a reference signal and generates an output signal when the differentiated light detector signal exceeds the reference value.
In an alternative embodiment, a second comparator circuit and a logic circuit are added to the circuit. The comparator circuit compares the light detector signal to a second reference signal and generates a second comparator output signal when the light detector signal exceeds the reference signal. The comparator output signal and the second comparator output signal are connected together in such a way as to provide a logical AND operation. The comparator and the second comparator outputs generate a trip signal only when both comparator outputs are activated.